Stratification and water quality of an abandoned opencast coal pit lake at Raniganj Coalfield Area,West Bengal,India

Stratification and assessment of the limnological conditions of an abandoned opencast coal pit (OCP), locally called Khadan, which is a potential water reserve located at the Raniganj‐Asansol Coalfield Areas of Burdwan, are the main objectives of this study. Water quality changes occurred with increasing depths in the OCP water and suggest a gradual and significant ecological restoration over time, which can create a condition of sustainable pisciculture. The physical and chemical variables of the OCP water changed with changing depths and seasons. The temperature and alkalinity decreased with depth throughout the year. The dissolved oxygen concentration exhibited a mixing characteristic in the summer and winter, but decreased with depth in the monsoon and postmonsoon seasons. The total hardness increased with depth in the monsoon season, being maximum in the winter (196.0 mg L^?1). The biochemical oxygen demand (BOD) and chemical oxygen demand (COD) increased with depth in the winter. The silicon concentration increased with depths, exhibiting a maximum value of 14.2 mg L^?1 in the winter and a minimum value of 1.24 mg L^?1 in the postmonsoon season. The ammonia‐nitrogen, nitrate‐nitrogen, potassium, chloride and sulphate concentrations only exhibited seasonal variations. The concentration of heavy metals (arsenic, chromium, cadmium, manganese) decreased with increasing depth. The fluoride concentration exhibited high concentrations in each OCP water layer. The results of this study indicated that limnological variables in thermally stratified lakes may be an important measure for better understanding the gradual process of developing an improved ecological conditions in the OCP and which can facilitate the possibility of pisciculture in the near future.